Who dies in their sleep?

In the book, Yitzchak's daughter, Tzipporah, is the one who dies in her sleep.
According to the text, Yitzchak is a butcher, and he is smitten with Gitl. For her part, however, Gitl shows little initial interest in Yitzchak. We are introduced to Yitzchak when he visits Shmuel and Gitl before Shmuel's wedding to Fayge.
Yitzchak is a widower and has two children, Reuven and Tzipporah. Shmuel teases Gitl that she would be far happier marrying Yitzchak than waiting for Avrom Morowitz to send for her in America. Both Reuven and Tzipporah are young; they are no more than three or four years old when we first meet them in the story.
Yitzchak and his children help Shmuel perform the farm chores while the latter is busy with wedding preparations. We do not learn much about Tzipporah from Yitzchak, but he does tell Gitl that his little girl is good at collecting the eggs that chickens lay. Later, we learn that Tzipporah quietly suffers in the death camps. Her brother, Reuven, is presumably sent to the crematorium.
In the story, Hannah is a comfort to Tzipporah in the camps. Later, Gitl and Hannah discover that Tzipporah died. The little girl looks as if she is sleeping with her thumb in her mouth. The only indication that she has died is the fly on her cheek. After the discovery, Gitl grieves that Yitzchak will have to be told of his beloved child's death.